75. Tricks about Pandas 1

75.1. Adding New Column from Groupby Results

import pandas as pd

Create a dataframe

df = pd.DataFrame( {'city':['London','London','Berlin','Berlin'], 'rent': [1000, 1400, 800, 1000]} )

df

	city	rent
0	London	1000
1	London	1400
2	Berlin	800
3	Berlin	1000

df['total'] = df.groupby('city').transform('sum')

df

	city	rent	total
0	London	1000	2400
1	London	1400	2400
2	Berlin	800	1800
3	Berlin	1000	1800

df['percent'] = df['rent']/df['total']

df

	city	rent	total	percent
0	London	1000	2400	0.416667
1	London	1400	2400	0.583333
2	Berlin	800	1800	0.444444
3	Berlin	1000	1800	0.555556